According to research by the Chartered Institute of Personnel and Development, ageism is becoming such a problem that there is a gap of only five years between the ages when people are considered too old or too young to do certain jobs.
Of the 600 workers and retired people surveyed, 20% said they had been discouraged from applying for a job because the advert implied an age restriction.
More than 33% of workers over 50 said they had experienced age discrimination at work, and 8% of staff under 35 said they had been told they were too young for a job.
Forthcoming age discrimination legislation could lead to a flood of employment tribunals in the IT industry, legal experts have warned. The research found that some employers have been accused by IT staff of discriminating against people in their fifties in favour of younger staff.
Under the EU rules, firms will not be able to specify an age for a job or restrict access to training opportunities for employees on grounds of age.
Dianah Worman, diversity adviser at the Chartered Institute of Personnel and Development, urged IT directors to work with human resources departments to tackle age discrimination.
"In IT the stereotype is that older people cannot learn new tricks, but when companies were preparing for the millennium bug, employers had to consult older, experienced staff who worked with the systems from the start," she said. "The business case for employing older workers seems more compelling, as they are more likely to stay in the job for longer - and the cost of replacing staff can be more than £3,500."
The Chartered Institute of Personnel and Development will publish a full paper on age discrimination later this month.